This invention relates generally to hydrodynamic retarders which control the speed of a descending load and such a system, for example, finds particular utility in oil well servicing and drilling rigs of the mobile type and wherein pipes are lowered into and raised out of oil wells. Some of these prior art rigs utilized water retarders which were of the essentially constant pressure type and required a water system and an associated water holding tank, and U.S. Pat. No. 3,072,224 issued Jan. 8, 1963 is an example of that type.
The torque capacity of a hydrodynamic retarder is a square function of the fluid velocity in the circuit, which in turn is essentially proportional to the rotational velocity of its rotor, as long as there is no fluid cavitation. Cavitation causes a reduction of the torque being transmitted, it can be suppressed by increasing the basic pressure in the hydraulic circuit. The higher the rotor speed, the higher the pressure that is required to prevent cavitation.
A constant pressure retarder has a torque versus speed performance curve that follows the square function law up to a certain speed after which the torque increase with speed is substantially less or even zero. A series of basic circuit pressure settings provide a series of torque versus speed curves that branch off the square function curve at different speeds and torque levels. Some prior art units, utilizing oil as the operating fluid, are equipped with circuit pressure adjustment features. These prior art units having constant but adjustable pressure settings, provide a means of matching the power absorption characteristics of the retarder to the load, but the torque versus speed relationship is undesirable for load lowering purposes. The poor torque response to speed changes in the cavitating range makes the device unsuitable for speed control of a constant load. Minute variations in the applied load, drive line resistance, circuit pressure and even fluid temperature can result in substantial speed variation for a given pressure setting. In other words, there is no definite intersection between the torque vs. speed curve produced by the load and the retarder torque curve. It is important to the operator of a load lowering system that the lowering velocity is predictable and repeatable.
Another disadvantage of the relatively flat torque versus speed curve for load lowering functions is the high power absorption at speeds below the desired speed. This means that the acceleration of the load is severely inhibited by the retarder and the time to reach the desired lowering velocity can be excessive. As a result, existing retarders are normally not used for light and medium load lowering operation.
U.S. Pat. No. 4,043,434 is an example of the prior art dealing with the design of the hydrodynamic retarder itself and deals with the shortcomings of the various prior art devices. This patent accomplishes torque control by restricting the internal circuit flow velocity with ring valves which is mechanically complicated and expensive.
Another prior at retarder control is shown in U.S. Pat. No. 3,863,739 issued Feb. 4, 1975 which utilizes a torque converter, hydrodynamic retarder and fluid circuitry in a system which has pressure sensing valves that sense the pressure differential across the inlet and outlet of the retarder for control purposes. A direct speed sensing device is not employed, but instead this patent advocates the pressure differential approach for control. That control system has certain shortcomings as to its ability to operate consistently at any speed and accurately, and the pressure differential is not a good indicator, particularly when cavitation exists in the retarder circuit. Furthermore, the manual control consisting of a series of solenoids which effect the level of pressure developed in the circuit is not infinitely variable.